I'm not sure re 6 months being a critical TTC milestone.
Looking around at friends & acquaintances, quite a few of them took a lot longer than 6 months to conceive, with most of them being 35 or younger.
For me, personally, we've now had regular unprotected intercourse for over 6 months and I'm gutted not to be pregnant yet.
We had a MMC 7 months ago.
Following the MMC I tried to cheer myself up by thinking that I would be pregnant again by the end of the year for sure.
I've been pregnant twice and both times there was no planning or timing involved at all, and we only had sex the one time in the month I fell pregnant.
For me, as its been 6 months and I am 38, am going back to the GP later this month to request some more investigations (bloods came back ok a couple of months ago)- I think I want to ask for an ultrasound scan to check all is looking ok.
I know it's completely normal not to conceive within 6 months, but sadly I don't feel that I have time on my side to just wait and see/relax & let it happen etc...so for me, passing the 6 months mark means I am now seriously worried and have kind of given up planning too much about baby no.2 as I don't know whether there will be one.